Fix redirects for governance pages (Bug 860532)

To atone for prematurely committing PR #836, I've updated the redirects for that PR.

The redirects have been moved from urls.py into global.conf, and a /b pass-through has been added.

I can't test the /b passthrough - but I think this should work (a single /b redirect for /about/governance* after the other redirects from the old .html URLs).